1. Field of the Invention
The present invention relates to an improved offset printer, and more particularly to an improved apparatus for high speed feeding and printing on cylindrical objects.
The present invention is particularly suitable in the manufacture of cylindrical containers, such as shotshell ammunition, where printing on the surface of corrugated, plastic cylindrical shells must be accomplished economically in a continuous run at relatively high speeds, interface readily with other high speed equipment in an automated manufacturing process, and yield acceptable high quality print appearance. While the invention disclosed herein is described generally in connection with shotshell ammunition manufacture, it will be readily apparent and is to be understood that the printing apparatus of the present invention can be used wherever high speed and quality printing on cylindrical objects, such as cans or other types of containers, is a desired goal.
2. Description of the Prior Art
High speed, yet economical, printing of acceptable quality is of particular concern in shotshell manufacture where present production techniques produce shells at very high rates. Quite obviously, printing apparatus unable to produce acceptable quality print on the cylindrical corrugated surface of the shell at the same output rate as the other manufacturing equipment defeats the economics of high speed automated production.
Prior art printing apparatus for shotshell printing has included conventional letterflex printers and hot stamp printers. The latter type generally produce very high quality print by means of a heated shell logo die used to print a pigmented carrier film firmly against the cylindrical shell surface. Crisp, clear letters which adhere well to the corrugated plastic surface are formed on areas where the hot die contacts the carrier film. Disadvantages of the hot stamping process include prohibitively high cost and low speed. The cost is approximately four times that of ink printing and the speed is severely limited by the time required for heat transfer. The speed required in high speed shell manufacture, however, is beyond the capability of present hot stamp printers.
Letterflex printers employed in shotshell printing, although not generally speed limited, produce print of low quality. The characters are often sloppy, distorted and smeared.
An example of such a conventional letterflex printer is schematically illustrated in FIG. 1. Paste-type ink is applied to the printing surface by a print roll 10. The latter is supplied with ink through continuous rolling contact with an ink roll 12. The print roll 10 includes a flexible printing plate or stamp 14 of rubber or composition material having a backward reading raised image. Ink applied to the raised surface by the ink roll 12 is transferred by direct contact of the flexible stamp 14 to the surface to be printed. In such printers, the plate is very sensitive to the thickness of the ink applied by the ink roll so that if the latter carries an ink film that is not uniform the resulting printed image will exhibit such non-uniformities present.
The prior art letterflex printer of FIG. 1 is shown printing on the corrugated surface of loaded shotshell ammunition at a point near the end of the production cycle. Shells 16 from a standard loader apparatus (not shown) are dropped into pockets of an indexing conveyor belt 18 which must stop intermittently to accommodate the printing operation. During the conveyor dwell, the shell at the printing station is lifted out of the belt by rollers 20 so as to be contacted and rotated by the print roll 10. A film of ink from the supply fountain 22 is deposited on an ink roll 12, which is formed of hardened steel, by an adjustable wiper blade 24. The ink is then transferred to the rubber stamp 14 on the print roll, which in turn transfers the ink image to the corrugated rib surface of the shell as the stamp rotates into direct contact with the shell.
The ink metering system of FIG. 1 has several disadvantages in addition to those described above. Ink 25 is applied through the small clearance gap between the hardened steel ink roll 12 and the wiper blade 24. These rigid boundaries are prone to severe streaking. Even minute lint and dust particles collecting at this point will form streaks in the ink film supplied to the ink roll. Moreover, the amount of ink released and applied to the roll varies greatly, due to the inability of accurately positioning the wiper blade relative to the ink roll.
The direct letterflex printing technique is also not well suited for this particular application due to the deeply corrugated surface of the shells. As shown in the magnified insert of FIG. 1A, a soft, compliant stamp is necessary to conform to the corrugated surface 16A in order to obtain complete printing on the surface. Such a stamp, however, has poor character definition and is subject to severe wear, and its durability is sharply reduced by the continuous printing operation. Moreover, the corrugated shells are unevenly rotated when driven by the hard surface portion of the steel print roll. This uneven rotation is further exaggerated by the stamp protruding from the print roll, resulting in speed differential which abruptly jolts the shell, causing smearing of the printed image.
Additional disadvantages in the above-described printing apparatus are due to the non-positive nature of its shell feeding. In the feeding mechanism shown in FIG. 1, shells from the standard loader (not shown) simply roll down a feed track 26 toward the indexing conveyor belt 18. A swinging door 28 in the area at the exit of the track provides the only means available for urging the shell into proper orientation in a pocket of the conveyor. This lack of positive feed control results in feed jams and skips.
Feed jams occur when the shell slips around sideways out of the conveyor pocket. Such jams are caused by a sudden jolt from the moving belt and will create a domino effect along the indexing conveyor. Jams can also severely damage the print roll.
Skips, on the other hand, occur when a shell does not fall into a pocket each time the conveyor belt indexes. Skips cause excessive ink build-up on the print roll, since the ink roll is continually applying ink to the former, which is not in turn transferring the applied ink due to the absence of a shell at the printing station. As a result, the next shell fed to the printing station after the skip will be contacted by the stamp with excessive ink, causing unclear, filled-in print image of generally poor quality.
Prior efforts have been made to solve some of the aforementioned deficiencies associated with offset printing by, for example, providing numerous rollers to uniformly distribute the ink onto the ink roll in order to form a continuous film for transfer to the offset roller. Such ink metering systems are commonly found in newspaper printing presses and are quite complex. They may include, for example, a fountain roll, a reciprocating ductor roll, and several distribution rolls to evenly apply the ink film onto a form roll, which in turn inks a plate cylinder carrying the image to be printed. Still others have included complex speed-reduction mechanisms for dealing with skips whereby, for example, the printer and/or the conveyor belt will completely stop when a skip occurs. Quite obviously, such equipment greatly reduces speed and increases costs, thus severely affecting the output of a high speed automated production process.